1. Field of the Invention
The disclosures herein generally relate to an image processing apparatus having a first image output device outputting first output data from original image data, and a second image output device outputting second output data from the original image data in such a way that the color tone of the first output data is reproduced in the second output data.
2. Description of the Related Art
Image output devices such as printers, displays, and the like, need to output image data according to pixel values of original image data. For this purpose, the color profile of an image output device may be updated, according to a comparison between pixel values in the original image data and pixel values in a printed material measured by a colorimeter. In a case of updating the color profile of an image output device, a method using a color chart is widely practiced. In this method, an image output device outputs a color chart whose original pixel values are known beforehand. Then, the printed color chart is measured by a colorimeter such as scanner. The known pixel values and the measured pixel values are compared. Finally, the color profile of the image output device is updated, according to the comparison result. Japanese Laid-open Patent Application No. 2009-177790, for example, discloses details.
There are two patterns of operations for updating the color profile of an image output device. Details are as follows, taking a printer as an example.
a) 1st Pattern: Adjusting Color Tone to a Reference Color Chart
In this method, a predetermined reference color chart is printed by the image output device. Each color patch in the color chart is measured by a colorimeter. The color profile of the image output device such as a printer is updated so that the difference between the measured value and the expected value will fall into a predetermined range.
b) 2nd Pattern: Adjusting Color Tone to a Reference Image Output Device
As an example of this method, a proofer may be used. A proofer is a calibration device, or a printer that can generate equivalent output as the calibration device. The color tone of the output image from a proofer is matched with the color tone of the output image from the image output device. In this case, the proofer and the image output device print color charts, respectively. Each color patch in the two printed color charts is measured by a colorimeter. The color profile of the proofer is updated so that differences between two measured values of patches fall into a predetermined range.
The previously used color profile update methods, however, have a problem in that these methods cannot be executed when a printed reference color chart is not available. As described above, in a case of matching the color tone of the output image from an image output device with the color tone of the output image from another image output device, both devices need to output the same color chart. In reality, a reference image output device may not be able to output a color chart, or another image output device whose color profile needs to be updated may not be able to obtain a printed material of the color chart from the reference image output device.
As an example of these cases, a commercial printing company may receive an order from a customer. The customer may request the commercial printing company to match the color tone of ordered printed materials with the output image obtained by the customer's printer. If the customer adopts proper color management, it is possible for the commercial printing company to satisfy the customer's request under the circumstances described above. Unfortunately, the customer may not be an expert of color management. Proper color management includes periodic calibration of image output devices, or the adoption of standard color management of image data such as an International Color Consortium profile.
If the reference color chart is not available and the customer does not adopt proper color management, the commercial printing company needs to match colors by hand. This work may be done through a trial and error process, which is time consuming and requires a lot of skill, experiences, and intuition of the operator of this work. Moreover, as the result of color matching is confirmed by using printed materials, this work consumes a lot of paper and the commercial printing company may suffer a loss due to the wasted paper.
Even if the commercial printing company adjusts the color tone with the output result from the customer's printer with an appropriate method, the color tone may not match well if the color space used for color matching is not appropriate.
To solve the inconvenience above, for example, color tone conversion could be applied to an original image before inputting to the target image output device, to obtain the same color tone output result as obtained with the reference image output device. Namely, to obtain the same output result, it could be possible to apply color tone conversion to the original image before inputting to the target image output device (Details will be described later with FIG. 5).
However, if the color space for executing color tone conversion is not appropriate to match the output result, it is difficult to obtain the same output result.
Namely, when executing color matching, it is necessary to use a color space that can appropriately represent the color difference. For example, if a color distribution in an image is confined in a narrow range, a color difference of a color with a few shades may not be extracted appropriately.
In other words, if a color space is available in which a color distribution in an image is dispersed in a wide range, colors having differences can be identified in a whole range of shades, which may result in a highly precise color matching.
Therefore, it is important for color tone conversion to select a color space in which a color distribution is dispersed in a wide range. If a color space in which a color distribution confined in a narrow range is adopted, error differences may be induced outside of the range although highly precise estimation may be possible within the range. In addition, within a range with a small number of pixels, color differences may disappear after color tone conversion.